MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephone and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more moveable membranes and a static backplate, with a respective electrode deposited on the membrane(s) and backplate, wherein one electrode is used for read-out/drive and the other is used for biasing. A substrate supports at least the membrane(s) and typically the backplate also. In the case of MEMS pressure sensors and microphones the read out is usually accomplished by measuring the capacitance between the membrane and backplate electrodes. In the case of transducers, the device is driven, i.e. biased, by a potential difference provided across the membrane and backplate electrodes.
FIGS. 1A and 1B show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 102 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 103 is mechanically coupled to a generally rigid structural layer or back-plate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1A the second electrode 103 is embedded within the back-plate structure 104.
The capacitive microphone is formed on a substrate 105, for example a silicon wafer which may have upper and lower oxide layers 106, 107 formed thereon. A cavity 108 in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is provided below the membrane, and may be formed using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110. A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110.
A plurality of acoustic holes 12 are arranged in the back-plate 4 so as to allow free movement of air molecules through the back plate, such that the second cavity 10 forms part of an acoustic volume with a space on the other side of the back-plate. The membrane 11 is thus supported between two volumes, one volume comprising cavities 9 and substrate cavity 8 and another volume comprising cavity 11 and any space above the back-plate. These volumes are sized such that the membrane can move in response to the sound waves entering via one of these volumes. Typically the volume through which incident sound waves reach the membrane is termed the “front volume” with the other volume, which may be substantially sealed, being referred to as a “back volume”.
In some applications the backplate may be arranged in the front volume, so that incident sound reaches the membrane via the acoustic holes 112 in the backplate 104. In such a case the substrate cavity 108 may be sized to provide at least a significant part of a suitable back-volume. In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use, i.e. the substrate cavity forms part of an acoustic channel to the membrane and part of the front volume. In such applications the backplate 4 forms part of the back-volume which is typically enclosed by some other structure, such as a suitable package.
It should also be noted that whilst FIGS. 1A and 1B show the backplate being supported on the opposite side of the membrane to the substrate, arrangements are known where the backplate is formed closest to the substrate with the membrane layer supported above it.
In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium or quiescent position. The distance between the membrane electrode 102 and the backplate electrode 103 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown).
The membrane layer and thus the flexible membrane of a MEMS transducer generally comprises a thin layer of a dielectric material—such as a layer of crystalline or polycrystalline material. The membrane layer may, in practice, be formed by several layers of material which are deposited in successive steps. Thus, the flexible membrane 101 may, for example, be formed from silicon nitride Si3N4 or polysilicon. Crystalline and polycrystalline materials have high strength and low plastic deformation, both of which are highly desirable in the construction of a membrane. The membrane electrode 102 of a MEMS transducer is typically a thin layer of metal, e.g. aluminium, which is typically located in the centre of the flexible membrane 101, i.e. that part of the membrane which displaces the most. It will be appreciated by those skilled in the art that the membrane electrode may be formed by depositing a metal alloy such as aluminium-silicon for example. The membrane electrode may typically cover, for example, around 40% of area of the membrane, usually in the central region of the membrane.
Thus, known transducer membrane structures are composed of two layers of different material—typically a dielectric layer (e.g. SiN) and a conductive layer (e.g. AlSi).
Typically the membrane layer 101 and membrane electrode 102 may be fabricated so as to be substantially planar in the quiescent position, i.e. with no pressure differential across the membrane, as illustrated in FIG. 1A. The membrane layer may be formed so as to be substantially parallel to the back-plate layer in this quiescent position, so that the membrane electrode 102 is parallel to the back-plate electrode 103. However, over time, the membrane structure may become deformed—e.g. as a consequence of relatively high or repeated displacement—so that it will not return to exactly the same starting position.
It will be appreciated that both the membrane and the membrane electrode will suffer intrinsic mechanical stress after manufacture. The composite membrane and membrane electrode structure is typically formed by deposition which takes place at high temperatures of around a few hundred degrees Celsius. On return to room temperature, and as a consequence of the membrane and membrane electrode having greatly different thermal coefficients of expansion, the two layers contract by different amounts. Since the two layers are intimately mechanically coupled together, thus preventing the dissipation of stress by independent mechanical contraction, thermal induced mechanical stress arises within the layers of the membrane and membrane electrode structure. Thus, even at equilibrium (when the pressure differential across the membrane is substantially zero) the composite structure will tend to deform as a result of the thermal induced stress. This is similar to the well-known operation of bi-metallic strip thermostat sensors.
The deformation of the membrane and membrane electrode structure may be further exacerbated as a result of additional stresses that arise when the structure is subject to the application of a voltage bias across the metal electrodes. This is illustrated in FIGS. 2a through 2c which show the equilibrium or quiescent position of a composite membrane structure under different circumstances. Specifically, FIG. 2a illustrates the relative arrangement between a backplate plate electrode 103, which is supported by a backplate structure 104, and a membrane electrode 102, which is deposited on the top surface of a membrane 101, when there is no voltage applied across the pair of electrodes. It will be appreciated that FIG. 2a illustrates the ideal situation in which the membrane electrode structure is substantially planar. However, as discussed above, even when the pressure differential across the membrane is substantially zero and without a bias voltage being applied, the membrane and membrane electrode structure will actually exhibit some deformation due to the thermal stresses induced at manufacture.
FIG. 2b illustrates the membrane and membrane electrodes structure when a voltage bias is applied across the pair of electrodes. Specifically, as illustrated in FIG. 2b, the application of a voltage bias causes the electrostatic deformation of the composite membrane structure. The resultant capacitance is defined as the operating capacitance Co of the transducer.
In FIG. 2c the longer-term effects of the electrostatic deformation are illustrated. Specifically, it will be appreciated that the electrostatic deformation of the membrane structure causes a stretching force to be exerted on the metal electrode layer. Thus, additional tensile stress arises within the metal electrode layer which leads to the lengthening or elongation of the metal. This elongation or additional tensile stress increasing the deformation in the membrane and membrane electrode over time. Thus, as illustrated by FIG. 2c, the distance between the backplate and membrane electrodes slowly decreases over time and, consequently, the capacitance at a time t (Ct) will be greater than the initial operating capacitance Co. This can lead to a DC offset in the measurement signal from such a transducer, as the capacitance at the quiescent position is not the same. Furthermore, for a.c. audio signals, the change in capacitance leads to a variation in the signal charge for a given acoustic stimulus, i.e. the acousto-electrical sensitivity of the microphone.
It will be appreciated that the equilibrium or quiescent position of the membrane structure comprising the membrane and the membrane electrode is affected by the thermal interface stress induced at the time of manufacture, and is further affected by additional tensile stresses arising during the use of the transducer. These tensile stresses may contribute to the occurrence of a drift in sensitivity over time. The level or degree of sensitivity drift is typically very small. However, more recent applications of MEMS microphones (e.g. the use of MEMS microphones within a beamforming array of microphones] may require new levels of performance stability.
The present disclosure invention relates to MEMS transducers and processes which seek to alleviate the occurrence of sensitivity drift, also known as creep, for example by providing a transducer which exhibits a reduced plastic deformation as compared to sheet electrode designs but which also demonstrate a more stable sensitivity or performance. In particular, examples described herein provide membrane electrode designs which seek to achieve a reduction in sensitivity drift over time.